While I'm perpetually showing up to places right on time—okay, I'm usually late—my wife is always early. But as she neared her pregnancy due date, it became clear that one of the greatest anxieties of childbirth comes at the very end. Will the labor start naturally on time, or will the baby be so late that induction or Caesarean section is necessary?

If the goal is a drug-free birth—as ours was—being told you have to be induced could force a dramatic reimagining of your birth plan. If the goal is a vaginal delivery, being told you will need a Caesarean could be heart-wrenching. Yet being past your due date is not only a common anxiety for pregnant women, it's an incredibly common situation.

The 40-week—or nine-month—pregnancy feels like an indelible part of our culture. Store shelves are lined with books that reference it, movies have been named for it, and serious medical decisions are made because of it.

The concept of a 40-week pregnancy was popularized nearly 200 years ago by Franz Naegele, director of a maternity hospital in Germany and author of a textbook for midwives. In a method called Naegele's Rule, he calculated the date of birth by adding 280 days to the date of last menstrual period.

Naegele borrowed this idea of a 280-day gestation from the 18th-century Dutch physician and botanist Herman Boerhaave, who created the modern teaching hospital. Boerhaave based his estimate of a 280-day gestation on evidence from the Bible that pregnancy lasts 10 lunar months.

There's just one snag in Boerhaave's theory and Naegele's Rule—most pregnancies are not exactly 280 days. Only about 5 percent of women give birth on their due date.

In The Numbers Game, journalist Michael Blastland and economist Andrew Dilnot explain how common misunderstandings of statistics affect our perception of the world. They use the 280-day pregnancy to explain the essential problem with averages. "Two facts about pregnancy suggest that the simple average will be misleading. First, some mothers give birth prematurely. Second, almost no one is allowed to go more than two weeks beyond the due date before being induced," they write. "The effect of this imbalance—we count very early births but prevent the very late ones—is to produce a lower average than if nature were left to its own devices."

Blastland and Dilnot cite a study in Sweden that found half of all babies were born by the 282nd day of pregnancy, while Day 283 saw more births than any other single day. "If most women have not had their baby until they are least two days overdue," they write, "and women are more likely to be three days overdue than anything else, it invites an obvious question: Are they really overdue?"


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